Composite structures offer so many advantages to modern aircraft that one aspect - their behaviour in crashes - may be under-researched
With all the attention aircraft manufacturers and aviation regulators devote to improving passenger survivability in airliner accidents is it conceivable there can be an area that is being ignored - or at least underplayed? Pointing to the increasing use of composites in aircraft structures, some experts believe insufficient research is being conducted into the behaviour of these materials in a crash.
The debate centres on the risks to crash survivors and rescue workers when fibre-reinforced plastics are broken in an impact or exposed to fire. Airservices Australia, after a two-year study of what its airport fire and rescue services will need to do to prepare for arrival of the Airbus A380 in 2008, has concluded that special procedures will be required to handle emergencies involving the latest generation of composites-laden airliners like the A380 and Boeing 7E7.
The question to be asked is whether such special procedures are warranted, or whether the aircraft manufacturers are justified in claiming that concerns over the safety of composite materials following a crash are exaggerated and not supported by either research data or operational experience.
There is no doubt that composites have massive advantages to offer in modern aircraft construction, and it can be argued that they do not merely reduce an aircraft's weight and so improve its efficiency, but also have a positive effect on safety. Among the positive arguments are the resistance of composites to corrosion and the fact that, while not completely maintenance-free because delamination can occur, they do not suffer fatigue in the way metals do.
Despite the obvious attractions of composites, the commercial aircraft industry has been relatively cautious in its adoption, because of the time and experience required to reduce manufacturing costs and build regulator and operator confidence in the materials' in-service maintainability and repairability. Military aircraft with composite primary structures were flying - and crashing - years before Airbus fitted first a carbonfibre tailfin to the A310. But adoption has accelerated in recent years, to the point where Boeing's 7E7 will be the first airliner with a composite fuselage.
Over the years, there has been considerable research into the crashworthiness of composite structures, focusing on the different impact-absorption characteristics of ductile metal and brittle but strong fibre-reinforced plastics. NASA has crash-tested complete all-composite business aircraft fuselages and the US Army has drop-tested all-composite helicopter airframes, but no-one has publicly conducted a large-scale crash test of composite airliner structure.
In the absence of such a transparent demonstration, the question must be raised about how well and how openly the issue of the post-crash safety of composites has been discussed. All the public knows is that whenever a modern combat aircraft crashes, the area is cordoned off, and one reason given is that smashed composites can produce airborne particles that can be harmful to humans and animals.
In this vacuum of real data, the allegations of experts who proclaim danger from particles released by broken composites are potentially frightening. The particles are described as "needle-like", with the ability to penetrate clothing and flesh, and to damage internal organs if inhaled.
Boeing says it knows of "no data" supporting the allegations. Airbus points out that its aircraft, with composite tailplanes, fins and flaps, have been in service for many years, but in accidents there has never been evidence of harm to rescue workers or survivors. Composites used in the A380 have all exceeded the requirements for flammability and noxious gases propagation, the company says.
The manufacturers may well be right. Perhaps Airservices Australia has taken its zeal for high standards further than strictly necessary, using the "better safe than sorry" philosophy. But even if it takes special circumstances for the risks it perceives to be realised - like a wind of a certain velocity to keep the dangerous composite particles airborne - an accident may happen one day when those conditions prevail. Is Airservices Australia to be the only organisation ready for them? The answer is that specific research is needed to clarify the risks, and that is the responsibility of the certificating authorities - the European Aviation Safety Agency and the US Federal Aviation Administration.
Source: Flight International
